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Abstract:

A wireless communication device communicates with, using a plurality of
antennas, a communication partner device including a plurality of
antennas. The wireless communication device includes a communication
unit, an interference intensity judgment unit, and a determination unit.
The communication unit communicates with the communication partner device
using the plurality of antennas and is configured to use a MIMO (multiple
input multiple output) scheme when transmitting a signal to the
communication partner device. The interference intensity judgment unit
judges, based on predetermined criteria, whether or not an intensity of
an interference wave included in a received signal at the communication
partner device is high. The determination unit prohibits, upon the
interference intensity judgment unit judging that the intensity of the
interference wave is high, the communication unit from using the MIMO
scheme when transmitting a signal.

Claims:

1. A wireless communication device that communications with, using a
plurality of antennas, a communication partner device including a
plurality of antennas, the device comprising: a communication unit that
communicates with the communication partner device using the plurality of
antennas and is configured to use a MIMO (multiple input multiple output)
scheme in transmitting a signal to the communication partner device; an
interference intensity judgment unit that judges, based on predetermined
criteria, whether or not an intensity of an interference wave included in
a received signal at the communication partner device is high; and a
determination unit that determines to prohibit, upon the interference
intensity judgment unit judging that the intensity of the interference
wave is high, the communication unit from using the MIMO scheme when
transmitting a signal.

2. The wireless communication device according to claim 1, wherein the
interference intensity judgment unit judges whether or not the intensity
of the interference wave is high based on a reception quality obtained
based on a received signal yet to be subjected to a demodulation process
in the communication partner device and a reception quality obtained
based on a received signal subjected to the demodulation process in the
communication partner device.

3. The wireless communication device according to claim 1, further
comprising a reception quality judgment unit that judges whether or not
the reception quality at the communication partner device is good based
on predetermined criteria, wherein the determination unit does not
prohibit the communication unit from using the MIMO scheme when
transmitting a signal in a case where the interference intensity judgment
unit judges that the intensity of the interference wave is high and in a
case where the reception quality judgment unit judges that the reception
quality is good, and the determination unit prohibits the communication
unit from using the MIMO scheme when transmitting a signal in a case
where the interference intensity judgment unit judges that the intensity
of the interference wave is high and in a case where the reception
quality judgment unit judges that the reception quality is not good.

4. (canceled)

5. (canceled)

6. A wireless communication device that communicates with, using a
plurality of antennas, a communication partner device including a
plurality of antennas, the communication partner device being configured
to use a MIMO (multiple input multiple output) scheme when transmitting a
signal to the wireless communication device, the wireless communication
device comprising a communication unit that communicates with the
communication partner device using the plurality of antennas and is
configured to, when receiving a signal from the communication partner
device, perform null steering using an algorithm that does not need
information regarding an interference wave for the reception directivity
at the plurality of antennas, wherein in a case where the communication
partner device transmits a signal without using the MIMO scheme, the
communication unit performs the null steering based on a known reference
signal from the communication partner device and then receives a data
signal from the communication partner device, and in a case where the
communication partner device transmits a signal using the MIMO scheme,
the communication unit does not perform the null steering based on a
known reference signal from the communication partner device and then
receives a data signal from the communication partner device.

7. A wireless communication device that communicates with, using a
plurality of antennas, a communication partner device including a
plurality of antennas, the communication partner device being configured
to use a MIMO (multiple input multiple output) scheme when transmitting a
signal to the wireless communication device, the wireless communication
device comprising: a communication unit that communicates with the
communication partner device using the plurality of antennas; an
interference intensity judgment unit that judges, based on predetermined
criteria, whether or not an intensity of an interference wave included in
a received signal at the communication unit is high; and a determination
unit that determines to prohibit, upon the interference intensity
judgment unit judging that the intensity of the interference wave is
high, the use of the MIMO scheme when the communication partner device
transmits a signal.

8. The wireless communication device according to claim 7, further
comprising: a reception processing unit that performs a demodulation
process on a received signal received by the communication unit; and a
reception quality acquisition unit that obtains a reception quality based
on a received signal yet to be subjected to the demodulation process and
obtains a reception quality based on a received signal subjected to the
demodulation process, wherein the interference intensity judgment unit
judges whether or not the intensity of the interference wave is high
based on the reception quality obtained by the reception quality
acquisition unit based on the received signal yet to be subjected to the
demodulation process and the reception quality obtained by the reception
quality acquisition unit based on the received signal subjected to the
demodulation process.

9. The wireless communication device according to claim 7, further
comprising a reception quality judgment unit that judges, based on
predetermined criteria, whether or not the reception quality obtained by
the reception quality acquisition unit is good, wherein in a case where
the interference intensity judgment unit judges that the intensity of the
interference wave is high but in a case where the reception quality
judgment unit judges that the reception quality is good, the
determination unit does not prohibit the use of the MIMO scheme when the
communication partner device transmits a signal, and in a case where the
interference intensity judgment unit judges that the intensity of the
interference wave is high and in a case where the reception quality
judgment unit judges that the reception quality is not good, the
determination unit prohibits the use of the MIMO scheme when the
communication partner device transmits a signal.

10. The wireless communication device according to claim 7, wherein the
communication unit is configured to, when receiving a signal from the
communication partner device, perform null steering using an algorithm
that does not need information regarding an interference wave for the
reception directivity at the plurality of antennas, in a case where the
communication partner device transmits a signal without using the MIMO
scheme, the communication unit performs the null steering based on a
known reference signal from the communication partner device and then
receives a data signal from the communication partner device, and in a
case where the communication partner device transmits a signal using the
MIMO scheme, the communication unit does not perform the null steering
based on the known reference signal from the communication partner device
and then receives a data signal from the communication partner device.

11. The wireless communication device according to claim 10, wherein in a
case where the communication partner device transmits a signal using the
MIMO scheme, the communication unit estimates a channel matrix of a
desired wave based on a known reference signal, which has been received
through the null steering based on the known reference signal from the
communication partner device, from the communication partner device and
performs a reception process on the data signal from the communication
partner device using the estimated channel matrix.

12. The wireless communication device according to claim 6, wherein in a
case where the communication partner device transmits a signal using the
MIMO scheme, the communication unit estimates a channel matrix of a
desired wave based on a known reference signal, which has been received
through the null steering based on the known reference signal from the
communication partner device, from the communication partner device and
performs a reception process on the data signal from the communication
partner device using the estimated channel matrix.

Description:

TECHNICAL FIELD

[0001] The present invention relates to a technique of performing
communication using a plurality of antennas.

BACKGROUND ART

[0002] Various techniques of radio communication have been conventionally
proposed. For example, Patent Document 1 discloses the technique of using
a MIMO (multiple input multiple output) scheme in communication between a
plurality of wireless communication devices.

[0004] The wireless communication system including a plurality of wireless
communication devices is desired to improve the communication performance
between the plurality of wireless communication devices.

[0005] The present invention therefore has been made in view of the above,
and an object thereof is to provide a technique capable of improving the
communication performance between a plurality of wireless communication
devices.

Means for Solving the Problem

[0006] A wireless communication device according to one aspect
communicates with, using a plurality of antennas, a communication partner
device including a plurality of antennas, which includes: a communication
unit that communicates with the communication partner device using the
plurality of antennas and is configured to use a MIMO (multiple input
multiple output) scheme when transmitting a signal to the communication
partner device; an interference intensity judgment unit that judges,
based on predetermined criteria, whether or not an intensity of an
interference wave included in a received signal at the communication
partner device is high; and a determination unit that determines to
prohibit, upon the interference intensity judgment unit judging that the
intensity of the interference wave is high, the communication unit from
using the MIMO scheme when transmitting a signal.

[0007] A wireless communication system according to one aspect includes
first and second wireless communication devices that communicate with
each other, wherein: the first wireless communication device includes: a
first communication unit that communicates with the second wireless
communication device using a plurality of antennas and is configured to
use a MIMO (multiple input multiple output) scheme when transmitting a
signal to the second wireless communication unit; an interference
intensity judgment unit that judges, based on predetermined criteria,
whether or not an intensity of an interference wave included in a
received signal at the second wireless communication device is high; and
a determination unit that determines to prohibit, upon the interference
intensity judgment unit judging that the intensity of the interference
wave is high, the first communication unit from using the MIMO scheme
when transmitting a signal; the second wireless communication device
includes a second communication unit that communicates with the first
wireless communication device using a plurality of antennas and is
configured to, when receiving a signal from the first wireless
communication device, perform null steering using an algorithm that does
not need information regarding an interference wave for the reception
directivity at the plurality of antennas; in a case where the first
wireless communication device transmits a signal without using the MIMO
scheme, the second communication unit performs the null steering based on
a known reference signal from the first wireless communication device and
then receives a data signal from the first wireless communication device;
and in a case where the first wireless communication device transmits a
signal using the MIMO scheme, the second communication unit does not
perform the null steering based on a known reference signal from the
first wireless communication device and then receives a data signal from
the first wireless communication device.

[0008] A wireless communication device according to one aspect
communicates with, using a plurality of antennas, a communication partner
device including a plurality of antennas, the communication partner
device being configured to use a MIMO (multiple input multiple output)
scheme when transmitting a signal to the wireless communication device,
the communication partner device including a communication unit that
communicates with the communication partner device using the plurality of
antennas and is configured to, when receiving a signal from the
communication partner device, perform null steering using an algorithm
that does not need information regarding an interference wave for the
reception directivity at the plurality of antennas, wherein: in a case
where the communication partner device transmits a signal without using
the MIMO scheme, the communication unit performs the null steering based
on a known reference signal from the communication partner device and
then receives a data signal from the communication partner device; and in
a case where the communication partner device transmits a signal using
the MIMO scheme, the communication unit estimates a channel matrix of a
desired wave based on the known reference signal, which has been received
through the null steering based on a known reference signal from the
communication partner device, from the communication partner device and
then performs a reception process on the data signal from the
communication partner device using the estimated channel matrix.

[0009] A wireless communication device according to one aspect
communicates with, using a plurality of antennas, a communication partner
device including a plurality of antennas, the communication partner
device being configured to use a MIMO (multiple input multiple output)
scheme when transmitting a signal to the wireless communication device,
and the wireless communication device includes: a communication unit that
communicates with the communication partner device using the plurality of
antennas; an interference intensity judgment unit that judges, based on
predetermined criteria, whether or not an intensity of an interference
wave included in a received signal at the communication unit is high; and
a determination unit that determines to prohibit, upon the interference
intensity judgment unit judging that the intensity of the interference
wave is high, the use of the MIMO scheme when the communication partner
device transmits a signal.

[0010] A wireless communication system according to one aspect includes
first and second wireless communication devices that communicate with
each other, wherein: the second wireless communication device
communicates with the first wireless communication device using a
plurality of antennas and is configured to use a MIMO (multiple input
multiple output) scheme when transmitting a signal to the first wireless
communication device; the first wireless communication device includes: a
communication unit that communicates with the second wireless
communication device using a plurality of antennas and is configured to
perform, when receiving a signal from the second wireless communication
device, null steering using an algorithm that does not need information
regarding an interference wave for the reception directivity at the
plurality of antennas; an interference intensity judgment unit that
judges, based on predetermined criteria, whether or not an intensity of
an interference wave included in a received signal at the communication
unit is high; and a determination unit that prohibits, upon the
interference intensity judgment unit judging that the intensity of the
interference wave is high, the use of the MIMO scheme when the second
wireless communication unit transmits a signal, in a case where the
second wireless communication device transmits a signal without using the
MIMO scheme; the communication unit performs the null steering based on a
known reference signal from the second wireless communication device and
then receives a data signal from the second wireless communication
device; and in a case where the second wireless communication device
transmits a signal using the MIMO scheme, the communication unit does not
perform the null steering based on the known reference signal from the
second wireless communication device and then receives a data signal from
the second wireless communication device.

Effects of the Invention

[0011] The present invention can improve the communication performance
between a plurality of wireless communication devices.

[0012] These and other objects, features, aspects and advantages of the
present invention will become more apparent from the following detailed
description of the present invention when taken in conjunction with the
accompanying drawings.

BRIEF DESCRIPTION OF DRAWINGS

[0013] FIG. 1 is a diagram showing a configuration of a wireless
communication system according to an embodiment of the present invention.

[0014] FIG. 2 is a diagram showing a configuration of a base station
according to the embodiment of the present invention.

[0015] FIG. 3 is a diagram showing a configuration of a communication
terminal according to the embodiment of the present invention.

[0016] FIG. 4 is a flowchart showing an operation of the wireless
communication system according to the embodiment of the present
invention.

[0017] FIG. 5 is a diagram for describing an operation of an interference
intensity judgment unit.

[0018] FIG. 6 is another diagram for describing the operation of the
interference intensity judgment unit.

[0019] FIG. 7 is a diagram showing exemplary radio resources that are used
when each antenna transmits reference signals and data signals.

[0020] FIG. 8 is a diagram showing a configuration of a modification of
the base station.

[0021] FIG. 9 is a flowchart showing an operation of a modification of the
wireless communication system.

[0022] FIG. 10 is a flowchart showing an operation of a modification of
the communication terminal.

[0023] FIG. 11 is a diagram showing a configuration of another
modification of the base station.

[0024] FIG. 12 is a flowchart showing an operation of the other
modification of the base station.

DESCRIPTION OF EMBODIMENTS

[0025] FIG. 1 is a diagram showing a configuration of a wireless
communication system 100 according to an embodiment of the present
invention. The wireless communication system 100 is a wireless
communication system adopting an orthogonal frequency division
multiplexing (OFDM) scheme such as worldwide interoperability for
microwave access (WiMAX) and long term evolution (LTE). In the OFDM
scheme, an OFDM signal obtained by combining a plurality of orthogonal
subcarriers is used. The wireless communication system 100 according to
this embodiment adopts, for example, a time division duplexing (TDD)
scheme as a duplexing scheme.

[0026] As shown in FIG. 1, the wireless communication system 100 includes
a plurality of base stations 1 being wireless communication devices. Each
base station 1 performs two-way radio communications with a plurality of
communication terminals 2 being wireless communication devices. The
plurality of communication terminals 2 include mobile communication
terminals such as cellular phones. A service area 10 of each base station
1 partially overlaps service areas 10 of the neighboring base stations 1.
FIG. 1 shows only four base stations 1, and accordingly, only two or
three neighboring base stations 1 are located for one base station 1. In
actuality, however, for example, six neighboring base stations 1 may be
located for one base station 1.

[0027] The plurality of base stations 1 are connected to a network (not
shown) and are capable of communicating with each other through this
network. A server device (not shown) is connected to the network. Each
base station 1 is capable of communicating with the server device through
the network.

[0028] <Configuration of Base Station>

[0029] FIG. 2 is a diagram showing a configuration of each base station 1.
The base station 1 according to the embodiment of the present invention
can use two types of schemes, the MIMO scheme and adaptive array antenna
scheme, when transmitting signals to the communication terminal 2. The
base station 1 uses the adaptive array antenna scheme when receiving
signals from the communication terminal 2. When receiving a signal from
the communication terminal 2 using the adaptive array antenna scheme, the
base station 1 obtains a reception weight to be set to a reception signal
for controlling the reception directivity at the base station 1, based on
a known reference signal from the communication terminal 2. When
transmitting signals to the communication terminal 2 using the adaptive
array antenna scheme, the base station 1 obtains a transmission weight to
be set in a transmission signal for controlling the transmission
directivity at the base station 1, from the reception weight.

[0030] Here, the MIMO scheme is a concept including the scheme for
improving a transmission speed (transmission throughput), such as spatial
division multiplexing (SDM), and the schemes for improving a
communication quality, such as space-time coding (STC) and space
frequency block coding (SFBC). The SDM may be merely referred to as
"spatial multiplexing (SM)". Also, the MIMO schemes that improve a
communication quality, such as STC and SFBC, may be referred to as
"transmission diversity". In this embodiment, the base station 1 uses,
for example, the SDM. The base station 1 may alternatively use the SDM
and the MIMO scheme that improves a communication quality, such as STC or
SFBC. Hereinafter, the SDM, STC, and SFBC may also be referred to as
"MIMO-SDM", "MIMO-STC", and "MIMO-SFBC".

[0031] In the adaptive array antenna scheme to be used in reception and
transmission by the base station 1, null steering, which uses an
algorithm that does not need the information of an interference wave
included in a reception signal at the base station 1, specifically, a
channel matrix (also referred to as a response vector or channel vector)
for the interference wave, is performed when a reception weight is
obtained. As the algorithm that does not need the information of an
interference wave, for example, a minimum mean squared error (MMSE) such
as least mean square (LMS) algorithm or recursive least-squares (RLS)
algorithm is adoptable. In the case where the LMS algorithm or RLS
algorithm is used, null steering and beamforming are both performed. In
this embodiment, the LMS algorithm or RLS algorithm is used, and
accordingly, null steering and beamforming are both performed for the
transmission directivity and reception directivity.

[0032] The interference wave included in the reception signal at the base
station 1 is an unnecessary signal from a communication device other than
the communication terminal 2 (communication terminal 2 within the service
area 10 of the base station 1) being a communication target for the base
station 1. The interference wave includes, for example, an unnecessary
signal from a communication terminal 2 that communicates with a
neighboring base station 1 located near this base station 1.

[0033] In the wireless communication system 100 according to this
embodiment, each base station 1 cannot identify a location of the
communication terminal 2 that communicates with the neighboring base
station 1 located near itself. For this reason, each base station 1
cannot identify through which transmission path (channel) an interference
wave from the communication terminal 2 communicating with the neighboring
base station 1 passes. Thus, in the adaptive array antenna scheme
according to this embodiment, null steering, which uses an algorithm that
does not need the information of an interference wave (channel matrix of
an interference wave), such as an LMS algorithm, is performed.

[0034] As shown in FIG. 2, the base station 1 includes a radio processing
unit 11 including an array antenna 13 composed of a plurality of antennas
12, and a controller 14 that controls the radio processing unit 11. The
radio processing unit 11 performs an amplification process,
down-conversion, A/D conversion process, and the like on each of a
plurality of received signals received by the array antenna 13, to
thereby generate and output a plurality of received signals of a
baseband.

[0035] The radio processing unit 11 performs a D/A conversion process,
up-conversion, amplification process, and the like on each of a plurality
of transmission signals of a baseband generated by the controller 14, to
thereby generate a plurality of transmission signals of a carrier band.
Then, the radio processing unit 11 respectively inputs the generated
plurality of transmission signals of the carrier band to the plurality of
antennas 12 constituting the array antenna 13. Accordingly, the
transmission signal is wirelessly transmitted from each antenna 12.

[0036] The radio processing unit 11 is provided with two antennas 12 in
the example of FIG. 2, which may be provided with two or more antennas
12.

[0037] The controller 14 is composed of a central processing unit (CPU), a
digital signal processor (DSP), a memory, and the like. In the controller
14, the CPU and DSP execute a program in the memory, whereby functional
blocks are formed, such as a transmission processing unit 140, a
reception processing unit 141, an interference intensity judgment unit
142, and a use prohibition determination unit 143.

[0038] The transmission processing unit 140 generates a transmission
signal and performs a modulation process or the like on the transmission
signal, to thereby generate a transmission signal of a baseband. As many
transmission signals of the baseband as the plurality of antennas 12
constituting the array antenna 13 are generated. The plurality of
transmission signals of the baseband generated by the transmission
processing unit 140 are input to the radio processing unit 11.

[0039] The transmission processing unit 140 generates a plurality of
transmission signals according to a transmission scheme used in the base
station 1. For example, in a case where the adaptive array antenna scheme
is used in the base station 1, the transmission processing unit 140
generates as many identical transmission signals as the antennas 12.
After that, the transmission processing unit 140 sets, for the obtained
plurality of transmission signals, a plurality of transmission weights
for controlling the transmission directivity of the array antenna 13.
Then, the transmission processing unit 140 performs a modulation process
or the like on the plurality of transmission signals in which
transmission weights have been respectively set, and thereafter, inputs
the plurality of transmission signals to the radio processing unit 11. As
a result, the base station 1 performs null steering and beamforming for
the transmission directivity of the array antenna 13, so that a
transmission signal is transmitted to the communication terminal 2.

[0040] Meanwhile, in a case where the base station 1 uses the MIMO-SDM,
the transmission processing unit 140 generates transmission signals of a
plurality of systems (a plurality of different transmission signals).
Then, the transmission processing unit 140 performs the modulation
process or the like on the obtained plurality of transmission signals,
and thereafter, inputs the plurality of transmission signals to the radio
processing unit 11. As a result, the base station 1 transmits the
transmission signals of the plurality of systems to the communication
terminal 2 using the same radio resources (frequency band and time slot),
which improves a transmission throughput of the base station 1.

[0041] In a case where the base station 1 uses the MIMO-STC or MIMO-SFBC,
the transmission processing unit 140 generates a plurality of different
transmission signals coded by, for example, alamouti coding. Then, the
transmission processing unit 140 performs the modulation process on the
obtained plurality of transmission signals, and then, inputs the
plurality of transmission signals to the radio processing unit 11. As a
result, the base station 1 achieves a transmission diversity effect,
which improves the communication quality between the base station 1 and
communication terminal 2.

[0042] The reception processing unit 141 sets, for a plurality of received
signals input from the radio processing unit 11, a plurality of reception
weights for controlling the reception directivity at the array antenna
13. The reception processing unit 141 combines the plurality of received
signals in which a plurality of reception weights have been respectively
set, to thereby generate a combined received signal. Then, the reception
processing unit 141 performs a demodulation process or the like on the
combined received signal that has been generated, to thereby obtain, for
example, control data and user data included in the combined received
signal.

[0043] The reception processing unit 141 calculates a plurality of
reception weights for controlling the reception directivity of the array
antenna 13, based on a known reference signal included in the received
signal from the communication terminal 2. The reception processing unit
141 calculates a plurality of reception weights using, for example, the
LMS algorithm or RLS algorithm. Then, the reception processing unit 141
respectively sets the calculated plurality of reception weights to data
signals included in the plurality of received signals output from the
radio processing unit 11. As a result, the base station 1 performs null
steering and beamforming based on the reference signal from the
communication terminal 2 and then receives a data signal of the
communication terminal 2.

[0044] Meanwhile, the transmission processing unit 140 calculates a
plurality of transmission weights for controlling the transmission
directivity of the array antenna 13 from the plurality of reception
weights obtained by the reception processing unit 141.

[0045] In the base station 1 according to this embodiment, a communication
unit 15, which communicates with the communication terminal 2 using the
array antenna 13, is formed of the radio processing unit 11, transmission
processing unit 140, and reception processing unit 141. The communication
unit 15 is capable of transmitting a signal using the MIMO scheme and
adaptive array antenna scheme, and receives a signal using the adaptive
array antenna scheme.

[0046] Based on the reception quality at the communication terminal 2
notified from the communication terminal 2, the interference intensity
judgment unit 142 judges whether or not the intensity of an interference
wave included in the received signal at the communication terminal 2 is
high based on predetermined criteria.

[0047] When the interference intensity judgment unit 142 judges that the
intensity of the interference wave included in the received signal at the
communication terminal 2 is high, the use prohibition determination unit
143 prohibits the use of the MIMO scheme in transmission of a signal to
this communication terminal 2. When the use prohibition determination
unit 143 prohibits the use of the MIMO scheme in transmission of a signal
to the communication terminal 2, the communication unit 15 does not use
the MIMO scheme but uses the adaptive array antenna scheme in
transmission of a signal to this communication terminal 2.

[0048] <Configuration of Communication Terminal>

[0049] FIG. 3 is a diagram showing a configuration of each communication
terminal 2. In a case of receiving a signal from the base station 1, the
communication terminal 2 according to the embodiment of the present
invention uses the adaptive array antenna scheme when this base station 1
does not use the MIMO scheme in transmitting a signal to this
communication terminal 2. Meanwhile, in a case of receiving a signal from
the base station 1, the communication terminal 2 is configured not to use
the adaptive array antenna scheme when the base station 1 uses the MIMO
scheme.

[0050] In a case of transmitting a signal to the base station 1, the
communication terminal 2 according to this embodiment uses the adaptive
array antenna scheme irrespective of whether or not this base station 1
uses the MIMO scheme in transmitting a signal to this communication
terminal 2.

[0051] Similarly in the adaptive array antenna scheme used by the base
station 1, in the adaptive array antenna scheme used in reception and
transmission by the communication terminal 2, when a reception weight is
obtained, null steering using an algorithm, which does not need the
information of an interference wave (channel matrix for an interference
wave) included in a received signal at the communication terminal 2, is
performed. The communication terminal 2 uses the LMS algorithm or RLS
algorithm in calculating a reception weight. Therefore, the communication
terminal 2 performs both null steering and beamforming in reception and
transmission. Here, the interference wave included in a received signal
at the communication terminal 2 refers to an unnecessary signal from a
communication device other than the base station 1 being a communication
target for this communication terminal 2. The interference wave includes,
for example, an unnecessary signal from a neighboring base station 1
located near this base station 1.

[0052] In the wireless communication system 100 according to this
embodiment, each communication terminal 2 cannot identify the location of
the neighboring base station 1 located near the base station 1
communicating with itself. For this reason, each communication terminal 2
cannot identify through which transmission path (channel), an
interference wave from a neighboring base station 1 located near the base
station 1 communicating with itself, passes. Thus, in the adaptive array
antenna scheme according to this embodiment, null steering using an
algorithm that does not need the information of an interference wave,
such as an LMS algorithm, is performed.

[0053] As shown in FIG. 3, the communication terminal 2 includes a radio
processing unit 21 including an array antenna 23 composed of a plurality
of antennas 22, and a controller 24 that controls the radio processing
unit 21. The radio processing unit 21 performs an amplification process,
down-conversion, A/D conversion process, and the like on each of a
plurality of received signals received by the array antenna 23, to
thereby generate and output a plurality of received signals of a
baseband.

[0054] The radio processing unit 21 performs a D/A conversion process,
up-conversion, amplification process, and the like on each of a plurality
of transmission signals of a baseband generated in the controller 24, to
thereby generate a plurality of transmission signals of a carrier band.
Then, the radio processing unit 21 respectively inputs the generated
plurality of transmission signals of the carrier band to the plurality of
antennas 22 constituting the array antenna 23. As a result, a
transmission signal is wirelessly transmitted from each antenna 22.

[0055] The controller 24 is composed of a CPU, DSP, memory, and the like.
In the controller 24, the CP and DSP execute a program in the memory,
whereby functional blocks are generated, such as a transmission
processing unit 240, a reception processing unit 241, and a reception
quality acquisition unit 242.

[0056] The transmission processing unit 240 generates as many identical
transmission signals as the antennas 22. After that, the transmission
processing unit 240 respectively sets, for the obtained plurality of
transmission signals, a plurality of transmission weights for controlling
the transmission directivity of the array antenna 23. Then, the
transmission processing unit 240 performs the modulation process or the
like on a plurality of transmission signals in which transmission weights
have been respectively set, to thereby generate a plurality of
transmission signals of a baseband. After that, the transmission
processing unit 240 inputs the generated plurality of transmission signal
of the baseband to the radio processing unit 21. As a result, the
communication terminal 2 performs null steering and beamforming on the
transmission directivity of the array antenna 23, and then transmits a
transmission signal to the base station 1.

[0057] In a case where the base station 1 does not use the MIMO scheme in
transmission, the reception processing unit 241 respectively sets, for a
plurality of received signals input from the radio processing unit 21, a
plurality of reception weights for controlling the reception directivity
at the array antenna 23. The reception processing unit 241 combines a
plurality of received signals in which reception weights have been
respectively set, to thereby generate a combined received signal. Then,
the reception processing unit 241 performs a demodulation process or the
like on the generated combined received signal, to thereby obtain, for
example, the control data and user data included in the combined received
signal. In this embodiment, the base station 1 notifies the communication
terminal 2 whether or not it uses the MIMO scheme in transmission to this
communication terminal 2.

[0058] The reception processing unit 241 calculates a plurality of
reception weights based on a known reference signal included in the
received signal from the base station 1. The reception processing unit
241 calculates a plurality of reception weights using, for example, the
LMS algorithm or RLS algorithm. Then, the reception processing unit 241
respectively sets the calculated plurality of reception weights to the
data signals included in a plurality of received signals output from the
radio processing unit 21. Accordingly, the communication terminal 2
performs null steering and beamforming based on the reference signal from
the base station 1 and receives a data signal transmitted from the base
station 1. The transmission processing unit 240 calculates a plurality of
transmission weights from the plurality of reception weights obtained by
the reception processing unit 241.

[0059] Meanwhile, in a case where the base station 1 uses the MIMO scheme
in transmission, the reception processing unit 241 performs the reception
process on the received signal from the base station 1 without using the
adaptive array antenna scheme. First, the reception processing unit 241
estimates a channel matrix in a transmission path between the array
antenna 23 of the communication terminal 2 to which the reception
processing unit 241 belongs and the array antenna 13 of the base station
1 being a communication target, based on known reference signals (complex
signals) included in a plurality of received signals output from the
radio processing unit 21. The channel matrix is formed of channel gains
(also referred to as channel coefficients) for the number obtained by
multiplying the number of a plurality of antennas 22 of the communication
terminal 2 and the number of a plurality of antennas 12 of the base
station 1. The reception processing unit 241 then performs, using the
estimated channel matrix, a reception process such as a demodulation
process on the data signals (complex signals) included in a plurality of
received signals output from the radio processing unit 21. The base
station 1 uses the MIMO-SDM in this embodiment, and thus, for example,
maximum likelihood detection (MLD), successive interference cancellation
(SIC), or MMSE is used in this reception process. This causes the
communication terminal 2 to obtain user data and control data transmitted
from the base station 1.

[0060] In the communication terminal 2 according to this embodiment, a
communication unit 25 that communicates with the base station 1 using the
array antenna 23 is formed of the radio processing unit 21, transmission
processing unit 240, and reception processing unit 241. The communication
unit 25 is capable of signal transmission using the adaptive array
antenna scheme and signal reception using the adaptive array antenna
scheme.

[0061] The reception quality acquisition unit 242 obtains a reception
quality of a signal from the base station 1, based on the received signal
output from the radio processing unit 21. In this embodiment, the
reception quality acquisition unit 242 obtains the reception quality
based on a received signal yet to be subjected to the demodulation
process and obtains the reception quality based on a received signal
subjected to the demodulation process. Hereinafter, the reception quality
that is obtained based on the received signal yet to be subjected to the
demodulation process is referred to as a "reception quality before
demodulation". Meanwhile, the reception quality that is obtained based on
the received signal subjected to the demodulation process is referred to
as a "reception quality after demodulation".

[0062] The reception quality acquisition unit 242 calculates, for example,
a carrier to interference and noise ratio (CINR) based on the received
signal of the baseband output from the radio processing unit 21, as the
reception quality before demodulation. The reception quality acquisition
unit 242 may calculate a received signal strength indicator (RSSI) as the
reception quality before demodulation.

[0063] The reception quality acquisition unit 242 calculates, for example,
a signal to interference and noise power ratio (SINR) as the reception
quality after demodulation, based on a received signal after the
demodulation process, namely a complex symbol (hereinafter, referred to
as a "demodulation complex symbol") for demodulating subcarriers included
in the received OFDM signal, the complex symbol being obtained by
performing a demodulation process on the received OFDM signal. An error
vector magnitude (EVM) may be calculated as the reception quality after
demodulation. Alternatively, the reception quality acquisition unit 242
may perform, for example, a decoding process on a demodulation complex
symbol to reproduce bit data and calculate a reception error rate based
on the bit data, thereby taking the resultant reception error rate as the
reception quality after demodulation.

[0064] The communication terminal 2 is configured such that when the
reception quality acquisition unit 242 obtains a reception quality before
demodulation and a reception quality after demodulation, the reception
processing unit 241 does not receive a received signal from the base
station 1 using the adaptive array antenna scheme.

[0065] The transmission processing unit 240 generates a transmission
signal including a reception quality notification signal for notifying
the reception quality before demodulation and reception quality after
demodulation obtained by the reception quality acquisition unit 242 and
input the generated transmission signal to the radio processing unit 21.
As a result, the communication terminal 2 transmits, to the base station
1, the reception quality notification signal for notifying the reception
quality before demodulation and reception quality after demodulation
obtained in the communication terminal 2.

[0066] In the base station 1, the reception quality notification signal
received by the radio processing unit 11 is input to the interference
intensity judgment unit 142. Based on the reception quality before
demodulation and reception quality after demodulation indicated by the
reception quality notification signal input, the interference intensity
judgment unit 142 judges whether or not the intensity (hereinafter,
referred to as "interference intensity") of an interference wave included
in the received signal at the communication terminal 2 that transmits
this reception quality notification signal is high based on predetermined
criteria. Then, in the base station 1, the use prohibition determination
unit 143 determines whether or not to prohibit the use of the MIMO scheme
based on the judgment results of the interference intensity judgment unit
142. Hereinafter, a series of processes in the wireless communication
system 100, which ranges from the communication terminal 2 obtaining the
reception quality to the base station 1 determining, based on the
reception quality, whether or not to prohibit the use of the MIMO scheme
in transmitting a signal to this communication terminal 2, is referred to
as a "MIMO use prohibition judging process".

[0068] FIG. 4 is a flowchart showing the MIMO use prohibition judging
process in the wireless communication system 100 according to the
embodiment of the present invention. As shown in FIG. 4, in the
communication terminal 2, in Step s1, the reception quality acquisition
unit 242 obtains the CINR as the reception quality before demodulation.
Next, in Step s2, the reception quality acquisition unit 242 obtains the
SINR as the reception quality after demodulation. Then, in Step s3, the
transmission processing unit 240 generates a reception quality
notification signal for notification of the reception quality before
demodulation (CINR) and reception quality after demodulation (SINR)
obtained by the reception quality acquisition unit 242. This reception
quality notification signal is transmitted to the base station 1 being a
communication target by the radio processing unit 21. The CINR and SINR
are notified the base station 1 from the communication terminal 2 as, for
example, a channel quality indicator (CQI). The communication terminal 2
may perform the process from Steps s1 to s3 based on an instruction from
the base station 1 or may perform the process regularly or irregularly
without an instruction from the base station 1.

[0069] In the base station 1, the radio processing unit 11 receives the
reception quality notification signal from the communication terminal 2,
and then, in Step s4, the interference intensity judgment unit 142 judges
whether or not the interference intensity at the communication terminal 2
is high based on the CINR and SINR indicated by the reception quality
notification signal. FIG. 5 is a diagram for describing an operation at
the interference intensity judgment unit 142.

[0070] The interference intensity judgment unit 142 deals with a
two-dimensional coordinate system 300 showing the CINR and SINR by
horizontal and vertical axes, respectively, as shown in FIG. 5. The
interference intensity judgment unit 142 compares a threshold straight
line 310 upward to the right, which is shown in the two-dimensional
coordinate system 300, and a position of a coordinate value 320
represented by the CINR and SINR indicated by the reception quality
notification signal in the two-dimensional coordinate system 300. The
interference intensity judgment unit 142 judges that the interference
wave intensity at the communication terminal 2 is low in a case where the
coordinate value 320 is located above the threshold straight line 310. In
other words, the interference intensity judgment unit 142 judges that the
interference wave intensity at the communication terminal 2 is low in a
case where on the threshold straight line 310, the SINR of the coordinate
value 320 is larger than the SINR with the CINR being identical to the
CINR of the coordinate value 320.

[0071] Meanwhile, in a case where the coordinate value 320 is located
below the threshold straight line 310 as shown in FIG. 5, the
interference intensity judgment unit 142 judges that the interference
wave intensity at the communication terminal 2 is high. In other words,
the interference intensity judgment unit 142 judges that the interference
wave intensity at the communication terminal 2 is high in a case where on
the threshold straight line 310, the SINR of the coordinate value 320 is
smaller than the SINR with the CINR being identical to the CINR of the
coordinate value 320.

[0072] The interference intensity judgment unit 142 may judge that the
interference wave intensity at the communication terminal 2 is low or the
interference wave intensity at the communication terminal 2 is high in a
case where the coordinate value 320 is located on the threshold straight
line 310.

[0073] As described above, the interference wave intensity at the
communication terminal 2 is judged to be low in the case where the
coordinate value 320 is located above the threshold straight line 310
upward to the right, or the interference wave intensity at the
communication terminal 2 is judged to be high in the case where the
coordinate value 320 is located below the threshold straight line 310
upward to the right. As a result, in a case of a poor reception quality
before the demodulation process at the communication terminal 2, the
interference wave intensity at the communication terminal 2 is judged to
be low if the reception quality after demodulation at the communication
terminal 2 is not very good. In a case of a good reception quality before
the demodulation process at the communication terminal 2, the
interference wave intensity at the communication terminal 2 is judged to
be high even if the reception quality after demodulation at the
communication terminal 2 is good to some extent.

[0074] From another perspective of the process by the interference
intensity judgment unit 142, it is conceivable that the interference
intensity judgment unit 142 could compare the SINR at the communication
terminal 2 with a threshold that varies in accordance with the CINR at
the communication terminal 2. Then, the interference intensity judgment
unit 142 judges that the interference wave intensity at the communication
terminal 2 is low if the SINR is larger than the threshold. Meanwhile,
the interference intensity judgment unit 142 judges that the interference
wave intensity at the communication terminal 2 is high if the SINR is
smaller than the threshold. The threshold to be compared with the SINR at
the communication terminal 2 is set to be larger as the CINR at the
communication terminal 2 becomes higher.

[0075] The threshold straight line 310 used by the interference intensity
judgment unit 142 is determined based on a predicted straight line 350
showing the relationship between the CINR and SINR at the communication
terminal 2, which is predicted in a case where the received signal at the
communication terminal 2 is assumed to include no interference wave. In
this embodiment, the threshold straight line 310 is set to be located
slightly below the predicted straight line 350.

[0076] Also in a case where the communication terminal 2 obtains the RSSI
as the reception quality before demodulation and obtains the SINR as the
reception quality after demodulation, whether or not the interference
wave intensity at the communication terminal 2 is high can be judged in a
similar manner. In this case, a two-dimensional coordinate system showing
the RSSI and SINR by horizontal and vertical axes, respectively, is used
in place of the two-dimensional coordinate system 300.

[0077] In a case where the communication terminal 2 obtains the CINR or
RSSI as the reception quality before demodulation and obtains the
reception error rate or EVM as the reception quality after demodulation,
not a threshold straight line upward to the right but a threshold curve
downward to the right is used. FIG. 6 is a diagram for describing the
operation of the interference intensity judgment unit 142 in this case.

[0078] As shown in FIG. 6, the interference intensity judgment unit 142
deals with a two-dimensional coordinate system 400 showing the CINR or
RSSI and the reception error rate or EVM by horizontal and vertical axes,
respectively. The interference intensity judgment unit 142 compares a
threshold curve 410 (more specifically, the threshold curve 410 having a
smaller vertical-axis value as the horizontal-axis value becomes larger)
downward to the right, which is shown in the two-dimensional coordinate
system 400, and a position of a coordinate value 420 represented by the
CINR (or RSSI) and the reception error rate (or EVM) indicated by the
reception quality notification signal in the two-dimensional coordinate
system 400. In a case where the coordinate value 420 is located above the
threshold curve 410 as shown in FIG. 6, the interference intensity
judgment unit 142 judges that the interference wave intensity at the
communication terminal 2 is high. Meanwhile, in a case where the
coordinate value 420 is located below the threshold curve 410, the
interference intensity judgment unit 142 judges that the interference
wave intensity at the communication terminal 2 is low. In a case where
the coordinate value 420 is located on the threshold curve 410, the
interference intensity judgment unit 142 may judge that the interference
wave intensity at the communication terminal 2 is low or may judge that
the interference wave intensity at the communication terminal 2 is high.

[0079] From another perspective of the above-mentioned process by the
interference intensity judgment unit 142, it is conceivable that the
interference intensity judgment unit 142 could compare the reception
error rate or EVM at the communication terminal 2 with a threshold that
varies in accordance with the CINR or RSSI at the communication terminal
2. Then, the interference intensity judgment unit 142 judges that the
interference wave intensity at the communication terminal 2 is low if the
reception error rate or EVM is smaller than the threshold. Meanwhile, the
interference intensity judgment unit 142 judges that the interference
wave intensity at the communication terminal 2 is high if the reception
error rate or EVM is larger than the threshold. The threshold to be
compared with the reception error rate or EVM at the communication
terminal 2 is set to be smaller as the CINR or RSSI at the communication
terminal 2 becomes higher.

[0080] The threshold curve 410 used by the interference intensity judgment
unit 142 is determined based on a predicted curve 450 showing the
relationship between the CINR (or RSSI) and the reception error rate (or
EVM) at the communication terminal 2, which is predicted in a case where
the received signal at the communication terminal 2 is assumed to include
no interference wave. In this embodiment, the threshold curve 410 is set
to be located slightly above the predicted curve 450.

[0081] As described above, the base station 1 according to this embodiment
judges whether or not the interference intensity at the communication
terminal 2 is high based on both of the reception quality before
demodulation and reception quality after demodulation at the
communication terminal 2. Accordingly, whether or not the interference
wave intensity at the communication terminal 2 is high can be judged more
properly. This will be described below.

[0082] As to the reception quality after demodulation such as the SINR at
the communication terminal 2, it may become poor for a large distance
between the communication terminal 2 and base station 1 even in a case
where the interference wave intensity at the communication terminal 2 is
low. Thus, also in a case of a poor reception quality after demodulation
at the communication terminal 2, the interference intensity at the
communication terminal 2 is low in some cases. Also, as to the reception
quality after demodulation at the communication terminal 2, it may become
good for a small distance between the communication terminal 2 and base
station 1 even in a case where the interference wave intensity at the
communication terminal 2 is high. Thus, also in a case of a good
reception quality after demodulation at the communication terminal 2, the
interference intensity at the communication terminal 2 is high in some
cases.

[0083] As described above, the interference intensity at the communication
terminal 2 may be low even for a poor reception quality after
demodulation at the communication terminal 2, or the interference
intensity at the communication terminal 2 may be high even for a good
reception quality after demodulation at the communication terminal 2.
Thus, unlike this embodiment, in the case where it is judged whether or
not the interference intensity at the communication terminal 2 is high
with reference to only the reception quality after demodulation at the
communication terminal 2, the judgment accuracy does not become very
high.

[0084] Meanwhile, the reception quality before demodulation such as the
CINR at the communication terminal 2 is good for a small distance between
the communication terminal 2 and base station 1, or the reception quality
before demodulation at the communication terminal 2 becomes poor for a
large distance between the communication terminal 2 and base station 1.

[0085] In this embodiment, thus, it is judged whether or not the
interference intensity at the communication terminal 2 is high not only
based on the reception quality after demodulation at the communication
terminal 2 but also based on the reception quality before demodulation at
the communication terminal 2. Specifically, for a poor reception quality
before the demodulation process at the communication terminal 2, the
distance between the communication terminal 2 and base station 1 is
judged to be large, and the interference wave intensity at the
communication terminal 2 is judged to be low even if the reception
quality after demodulation at the communication terminal 2 is not very
good. Meanwhile, for a good reception quality before the demodulation
process at the communication terminal 2, the distance between the
communication terminal 2 and base station 1 is judged to be small, and
the interference wave intensity at the communication terminal 2 is judged
to be high even if the reception quality after demodulation at the
communication terminal 2 is good to some extent. This allows for correct
judgment whether or not the interference wave intensity at the
communication terminal 2 is high.

[0086] Referring back to FIG. 4, when the interference intensity judgment
unit 142 judges that the interference intensity at the communication
terminal 2 is high in Step s4, in Step s5, the use prohibition
determination unit 143 prohibits the use of the MIMO scheme when
transmitting a signal to the communication terminal 2. Meanwhile, when
the interference intensity judgment unit 142 judges that the interference
intensity at the communication terminal 2 is low (is not high) in Step
s4, the use prohibition determination unit 143 does not prohibit the use
of the MIMO scheme when transmitting a signal to the communication
terminal 2.

[0087] When the use prohibition determination unit 143 prohibits the use
of the MIMO scheme when transmitting a signal to the communication
terminal 2, the communication unit 15 does not use the MIMO scheme but
use the adaptive array antenna scheme when transmitting a signal to the
communication terminal 2.

[0088] While the communication unit 15 uses the adaptive array antenna
scheme in transmission when not using the MIMO scheme in transmission in
this embodiment, the communication unit 15 may perform omni-transmission
without using the adaptive array antenna scheme.

[0089] <Method of Adjusting Transmission Throughput at Base Station>

[0090] Next, a method of adjusting a transmission throughput at the base
station 1 will be described. In the wireless communication system 100
according to this embodiment, M (M≧2) MCSs which have different
combinations of modulation schemes and code rates are defined. In the
LTE, 29 MCSs are defined. The M MCSs are respectively provided with ranks
from stage zero to stage (M-1), and as the rank becomes higher, a
momentary transmission throughput of the base station 1, which is
determined in accordance with the combination of a modulation scheme and
a code rate in the MCS corresponding to the rank, becomes higher. The
transmission processing unit 140 determines an MCS to be applied to the
transmission signal that is transmitted from the communication unit 15 to
the communication terminal 2, from the M MCSs based on the CQI from the
communication terminal 2. In other words, the transmission processing
unit 140 determines an MCS to be applied to the transmission signal that
is transmitted from the communication unit 15 to the communication
terminal 2, based on the reception quality at the communication terminal
2. As the reception quality at a communication terminal 2 becomes better,
the transmission processing unit 140 applies an MCS of a higher rank to
the transmission signal to be transmitted to the communication terminal
2.

[0091] When the transmission processing unit 140 determines an MCS to be
applied to the transmission signal to the communication terminal 2, the
communication unit 15 notifies the communication terminal 2 of this MCS.
At the communication terminal 2, the reception processing unit 241
performs a reception process according to the MCS notified from the base
station 1 on a received signal from the base station 1, to thereby obtain
data included in the received signal.

[0092] When notifying a communication terminal 2 of an MCS to be applied
to a transmission signal that is transmitted to the communication
terminal 2, the base station 1 notifies whether or not it uses the MIMO
scheme when transmitting a signal to the communication terminal 2.

[0093] The transmission processing unit 140 adjusts an MCS to be applied
to a transmission signal. The method of adjusting an MCS will be descried
below.

[0094] In this embodiment, when the communication unit 25 receives a
signal from the base station 1 in each communication terminal 2, the
reception processing unit 241 generates ACK/NACK information indicating
whether or not it has properly obtained the data included in the received
signal. The ACK/NACK information is included in the transmission signal
and is then notified the base station 1 from the communication unit 25.
The transmission processing unit 140 of the base station 1 observes the
ACK/NACK information notified from the communication terminal 2 and
calculates a reception error rate at the communication terminal 2. Then,
in a case where the reception error rate for the communication terminal 2
is high or low, the transmission processing unit 140 changes an MCS to be
applied to a transmission signal that is transmitted to this
communication terminal 2. For example, in a case of a high reception
error rate for a communication terminal 2, namely in a case of a
reception error rate larger than a first threshold, the transmission
processing unit 140 decreases, by one, the rank of the MCS applied to the
transmission signal that is transmitted to the communication terminal 2.
In a case of a low reception error rate for a communication terminal 2,
namely in a case of a reception error rate smaller than a second
threshold (<first threshold), the transmission processing unit 140
increases, by one, the rank of the MSC applied to the transmission signal
to be transmitted to the communication terminal 2.

[0095] As described above, for a high reception error rate at a
communication terminal 2, the transmission processing unit 140 decreases,
by one, the rank of the MCS to be applied to the transmission signal that
is transmitted to the communication terminal 2. For a low reception error
rate at a communication terminal 2, the transmission processing unit 140
increases, by one, the rank of the MCS to be applied to the transmission
signal that is transmitted to the communication terminal 2. Through the
above, the reception error rate at the communication terminal 2 falls
within a predetermined range.

[0096] While the base station 1 obtains a reception error rate at the
communication terminal 2 in the example above, as described above, the
communication terminal 2 may obtain a reception error rate as the
reception quality after demodulation and notify the base station 1. In
this case, the base station 1 may determine the rank of an MCS to be
applied to a transmission signal to a communication terminal 2 based on a
reception error rate notified from the communication terminal 2.

[0097] In a case where the base station 1 according to this embodiment
starts communication with a communication terminal 2, first, the
communication unit 15 starts communication with the communication
terminal 2 without using the MIMO scheme. Then, in a case where, as a
result of the adjustment of an MCS to be applied to a transmission signal
to the communication terminal 2 by the transmission processing unit 140,
the rank of the MCS reaches the highest rank, namely (M-1) rank, the
reception error rate at the communication terminal 2 is smaller than a
second threshold, and the use of the MIMO scheme in transmission of a
signal to the communication terminal 2 is not prohibited, the
communication unit 15 uses the MIMO scheme (MIMO-SDM) when transmitting a
signal to the communication terminal 2 for further improving the
transmission throughput of the base station 1. Hereinafter, the
transmission scheme for transmitting a signal without using the MIMO
scheme may also be referred to as a "non-MIMO scheme".

[0098] The communication unit 15 that transmits a signal to a
communication terminal 2 using the MIMO scheme avoids using the MIMO
scheme in transmitting a signal to the communication terminal 2 when the
reception error rate at the communication terminal 2 exceeds the first
threshold. In other words, the communication unit 15 switches the
transmission scheme from the MIMO scheme to the non-MIMO scheme. Then, in
a case where the rank of an MCS to be applied to a transmission signal to
the communication terminal 2 is the highest rank, the communication unit
15 uses the MIMO scheme (MIMO-SDM) when transmitting a signal to the
communication terminal 2 when the reception error rate at the
communication terminal 2 falls below the second threshold. In other
words, the communication unit 15 switches the transmission scheme from
the non-MIMO scheme to the MIMO scheme. Hereinafter, the base station 1
operates in a similar manner.

[0099] As described above, the base station 1 combines an MCS to be
applied to a transmission signal to the communication terminal 2 with
whether or not to use the MIMO scheme (MIMO-SDM) when transmitting a
signal to the communication terminal 2, to thereby properly adjust a
transmission throughput for the communication terminal 2.

[0100] In a case where the use prohibition determination unit 143
prohibits the use of the MIMO scheme when transmitting a signal to a
communication terminal 2, the communication unit 15 does not use the MIMO
scheme when transmitting a signal to the communication terminal 2 even in
a case where an MCS to be applied to a transmission signal to the
communication terminal 2 reaches the highest rank and thus the reception
error rate at the communication terminal 2 is smaller than the second
threshold. In other words, the communication unit 15 does not change the
transmission scheme from the non-MIMO scheme to the MIMO scheme.

[0101] When communicating with a communication terminal 2, the base
station 1 may first use the transmission diversity such as MIMO-STC or
MIMO-SFBC to transmit a signal to the communication terminal 2. Then,
when the rank of the MCS to be applied to the transmission signal to the
communication terminal 2 reaches the highest rank, the base station 1 may
use the MIMO-SDM in transmission to the communication terminal 2.

[0102] As described above, for a high intensity of an interference wave
included in a received signal at a communication terminal 2, the base
station 1 according to this embodiment prohibits the use of the MIMO
scheme when transmitting a signal to the communication terminal 2. For a
high interference intensity at a communication terminal 2, the intensity
of the interference wave included in the reference signal from the base
station 1, which is received by the communication terminal 2, becomes
higher, and accordingly, the accuracy of a channel matrix that is
estimated by the communication terminal 2 based on the reference signal
degrades. Therefore, for a high interference intensity at the
communication terminal 2, when a signal is transmitted to the
communication terminal 2 using the MIMO scheme, the communication
terminal 2 performs a reception process on a data signal from the base
station 1 based on a channel matrix whose accuracy is not good and thus
cannot properly obtain the data included in the data signal. In other
words, the reception error rate at the communication terminal 2
increases. In this embodiment, the use of the MIMO scheme when
transmitting a signal to a communication terminal 2 is prohibited in a
case where the interference intensity at the communication terminal 2 is
high, which does not cause such a problem. As a result, the communication
terminal 2 can properly obtain data included in the data signal from the
base station 1. In other words, the reception error rate at the
communication terminal 2 decreases. This results in an improvement in
communication performance between the base station 1 and communication
terminal 2.

[0103] In a case where the base station 1 adjusts a transmission
throughput as described above, if a communication terminal 2 has a high
interference wave intensity, the use of the MIMO scheme when transmitting
a signal to the communication terminal 2 is not prohibited. In this case,
the MCS to be applied to the transmission signal to a communication
terminal 2 reaches the highest rank in a situation where the interference
intensity at the communication terminal 2 is high, and the transmission
scheme of the base station 1 is switched from the non-MIMO scheme to the
MIMO scheme in a case where the reception error rate at the communication
terminal 2 is smaller than the second threshold. As a result, the
communication terminal 2 performs a reception processes on a signal from
the base station 1 based on a channel matrix having poor accuracy. This
results in an increase in reception error rate at the communication
terminal 2.

[0104] Upon an increase in reception error rate at the communication
terminal 2, the base station 1 switches the transmission scheme from the
MIMO scheme to the non-MIMO scheme. Accordingly, the communication
terminal 2 receives a signal from the base station 1 using the adaptive
array antenna scheme. In a case where the communication terminal 2
receives a signal from the base station 1 using the adaptive array
antenna scheme, the intensity of the interference wave included in the
received signal can be reduced, leading to a decrease in reception error
rate at the communication terminal 2.

[0105] Upon a decrease in reception error rate at the communication
terminal 2, the base station 1 again switches the transmission scheme
from the non-MIMO scheme to the MIMO scheme. Switching of the
transmission scheme from the non-MIMO scheme to the MIMO scheme by the
base station 1 increases a reception error rate at the communication
terminal 2 in a similar manner. As a result, the base station 1 switches
the transmission scheme from the MIMO scheme to the non-MIMO scheme.
After that, the base station 1 repeats a similar operation.

[0106] As described above, if the use of the MIMO scheme is not prohibited
when transmitting a signal to a communication terminal 2 in a case of a
high interference wave intensity at the communication terminal 2,
switching from the non-MIMO scheme to the MIMO scheme and switching from
the MIMO scheme to the non-MIMO scheme frequently occur in an alternate
manner in the base station 1, which destabilizes the operation of the
base station 1. This results in a decrease in communication performance
between the base station 1 and communication terminal 2.

[0107] In this embodiment, for a high interference wave intensity at the
communication terminal 2, the base station 1 prohibits the use of the
MIMO scheme when transmitting a signal to the communication terminal 2.
This prevents the frequent occurrence of switching from the non-MIMO
scheme to the MIMO scheme and switching from the MIMO scheme to the
non-MIMO scheme in an alternate manner in the base station 1. This
prevents the base station 1 from operating unstably. As a result, the
communication performance between the base station 1 and communication
terminal 2 can be improved.

[0108] The communication terminal 2 according to this embodiment does not
receive a data signal from the base station 1 using the adaptive array
antenna scheme in a case where the base station 1 uses the MIMO scheme in
transmission to the communication terminal 2, which allows the
communication terminal 2 to properly receive a signal transmitted by the
base station 1 using the MIMO scheme. This will be described below in
detail.

[0109] When the base station 1 transmits a signal using the MIMO scheme,
the transmission of a plurality of different reference signals from a
plurality of antennas 12 of the base station 1 using the same resources
(same frequency band and same transmission time slot) is avoided such
that the communication terminal 2 can properly estimate a channel matrix
using a reference signal from the base station 1.

[0110] Meanwhile, as to a data signal transmitted by the base station 1
using the MIMO scheme, a plurality of different data signals are
transmitted from the plurality of antennas 12 using the same radio
resources. For example, in the MIMO-SDM, a plurality of different data
signals that respectively belong to a plurality of different systems are
transmitted from the plurality of antennas 12 using the same radio
resources. In the MIMO-STC or MIMO-SFBC, a data signal belonging to one
system and a complex conjugate signal belonging to the other system are
transmitted using the same radio resources from the plurality of antennas
12.

[0111] FIG. 7 is a diagram showing exemplary radio resources that are used
when each antenna 12 transmits reference signals and data signals. FIG. 7
shows, by circles, unit radio resources 500 including one subcarrier in
the frequency direction and including one OFDM symbol period in the time
direction. In FIG. 7, the circles indicating the unit radio resources 500
that are used in transmission of reference signals are diagonally shaded,
the circles indicating the unit radio resources 500 that are used in
transmission of data signals are vertically shaded, and the circles
indicating the unit radio resources 500 that are not used in signal
transmission are not hatched. FIG. 7 shows one antenna 12 included in the
base station 1 as a "first antenna" and the other antenna 12 included in
the base station 1 as a "second antenna". The left side of FIG. 7 shows
exemplary radio resources that are used when the first antenna transmits
signals, and the right side of FIG. 7 shows exemplary radio resources
that are used when the second antenna transmits signals.

[0112] In the example of FIG. 7, the second antenna does not transmit
signals using the unit radio resources 500 same as the unit radio
resources 500 that are used in transmission of reference signals by the
first antenna, and the first antenna does not transmit signals using the
unit radio resources 500 same as the unit radio resources 500 that are
used in transmission of reference signals by the second antenna. In other
words, a reference signal that is transmitted from the base station 1
using a certain unit radio resource 500 is transmitted from only one of
the first and second antennas.

[0113] Unlike the example of FIG. 7, in the base station 1, the same
plurality of reference signals may be transmitted using the same radio
resources from the plurality of antennas 12.

[0114] As described above, in the base station 1, a mode of transmitting a
reference signal for estimating a channel matrix and a mode of
transmitting a data signal differ from each other when transmitting a
signal using the MIMO scheme.

[0115] Meanwhile, in a case where the communication terminal 2 properly
receives, using the adaptive array antenna scheme, data signals
transmitted by the base station 1 using the MIMO scheme, the
communication terminal 2 needs to control the reception directivity at
the plurality of antennas 22 to individually receive a plurality of
different data signals transmitted from the plurality of antennas 12
included in the base station 1. In other words, for each of a plurality
of antennas 12 of the base station 1, the communication terminal 2 needs
to direct a beam toward a data signal transmitted from one antenna 12 and
direct null toward a data signal transmitted from the other antenna 12.

[0116] However, in the base station 1, a mode of transmitting a reference
signal and a mode of transmitting a data signal differ from each other
when the MIMO scheme is used. Thus, even if performing null steering for
the reception directivity at the plurality of antennas 22 based on the
reference signal transmitted from the base station 1, for each of a
plurality of antennas 12 included in the base station 1, the
communication terminal 2 cannot direct a beam toward a data signal
transmitted from one antenna 12 and direct null toward a data signal
transmitted from the other antenna 12. Therefore, the communication
terminal 2 cannot properly receive, using the adaptive array antenna
scheme, a data signal transmitted from the base station 1 using the MIMO
scheme.

[0117] In the example of FIG. 7, in the base station 1, when a reference
signal is transmitted from one antenna 12, a signal is not transmitted
from the other antenna 12. For this reason, even if performing null
steering and beamforming based on a reference signal transmitted from one
antenna 12, the communication terminal 2 can direct a beam toward a data
signal transmitted from the one antenna 12 but cannot direct null toward
a data signal transmitted from the other antenna 12. The communication
terminal 2 accordingly receives the data signal transmitted from one
antenna 12 of the base station 1 as well as the data signal transmitted
from the other antenna 12 of the base station 1 as an unnecessary signal,
and cannot properly receive the data signal transmitted from the one
antenna 12.

[0118] This embodiment is therefore configured such that the communication
terminal 2 does not receive a data signal from the base station 1 using
the adaptive array antenna scheme in a case where the base station 1 uses
the MIMO scheme in transmission to this communication terminal 2. This
enables the communication terminal 2 to properly receive a signal
transmitted by the base station 1 using the MIMO scheme. This results in
an improvement in communication performance between the base station 1
and communication terminal 2.

[0119] <Various Modifications>

[0120] <First Modification>

[0121] In the example above, in a case where the interference intensity at
a communication terminal 2 is high, a base station 1 immediately
prohibits the use of the MIMO scheme in transmission to the communication
terminal 2. However, the base station 1 may prohibit the use of the MIMO
scheme in transmission to this communication terminal 2 for the first
time in a case where the interference intensity at a communication
terminal 2 is high and also in a case where the reception quality at the
communication terminal 2 is not good (is poor). This modification will be
described below.

[0122] FIG. 8 is a block diagram showing a configuration of a base station
1 included in a wireless communication system 100 according to this
modification. As shown in FIG. 8, in the base station 1 according to this
modification, a controller 14 further includes a reception quality
judgment unit 144 as a functional block. The reception quality judgment
unit 144 judges whether or not the reception quality at a communication
terminal 2 is good based on predetermined criteria.

[0123] FIG. 9 is a flowchart showing a MIMO use prohibition judging
process in the wireless communication system 100 according to this
modification. As shown in FIG. 9, in the MIMO use prohibition judging
process according to this modification, Steps s1 to 3 described above are
performed. After that, in Step s4, an interference intensity judgment
unit 142 of the base station 1 judges that the interference intensity at
the communication terminal 2 is not high, and then, a use prohibition
determination unit 143 does not prohibit the use of the MIMO scheme when
transmitting a signal to the communication terminal 2, whereby the MIMO
use prohibition judging process ends.

[0124] Meanwhile, in Step s4, the interference intensity judgment unit 142
judges that the interference intensity at the communication terminal 2 is
high, and then, in Step s11, the reception quality judgment unit 144 of
the base station 1 judges whether or not the reception quality at the
communication terminal 2 is good based on predetermined criteria. The
reception quality at the communication terminal 2 here is not the
reception quality used in Step s4 but the reception quality at the
communication terminal 2 after the interference intensity judgment unit
142 judges that the interference intensity at the communication terminal
2 is high. In other words, the reception quality at a communication
terminal 2 refers to the reception quality at the communication terminal
2 when the interference intensity at the communication terminal 2 is
high.

[0125] In Step s11, the reception quality before demodulation obtained in
the communication terminal 2 may be used or the reception quality after
demodulation obtained in the communication terminal 2 may be used. In a
case where, for example, the CINR obtained by the communication terminal
2 is used in Step s11, the reception quality judgment unit 144 compares
the CINR obtained by the communication terminal 2 with a predetermined
threshold. Then, the reception quality judgment unit 144 judges that the
reception quality at the communication terminal 2 is good in a case where
the CINR is larger than the predetermined threshold or judges that the
reception quality at the communication terminal 2 is not good (is poor)
in a case where the CINR obtained by the communication terminal 2 is
smaller than the predetermined threshold. In a case where the CINR
matches the predetermined threshold, the reception quality judgment unit
144 may judge that the reception quality at the communication terminal 2
is good or is not good.

[0126] In a case where, for example, the EVM obtained by the communication
terminal 2 is used in Step s11, the reception quality judgment unit 144
compares the EVM obtained by the communication terminal 2 with a
predetermined threshold. The reception quality judgment unit 144 judges
that the reception quality at the communication terminal 2 is good in a
case where the EVM obtained by the communication terminal 2 is smaller
than the predetermined threshold or judges that the reception quality at
the communication terminal 2 is not good (is poor) in a case where the
EVM obtained by the communication terminal 2 is larger than the
predetermined threshold. In a case where the EVM matches the
predetermined threshold, the reception quality judgment unit 144 may
judge that the reception quality at the communication terminal 2 is good
or is not good.

[0127] In Step s11, the reception quality judgment unit 144 may calculate
the reception error rate for the communication terminal 2 based on the
ACK/NACK information notified from the communication terminal 2 to judge
whether or not the reception quality at the communication terminal 2 is
good based on the calculated reception error rate.

[0128] When it is judged in Step s11 that the reception quality at the
communication terminal 2 is good, the use prohibition determination unit
143 does not prohibit the use of the MIMO scheme when transmitting a
signal to this communication terminal 2, whereby the MIMO use prohibition
judging process ends.

[0129] Meanwhile, when it is judged in Step s11 that the reception quality
at the communication terminal 2 is not good, the use prohibition
determination unit 143 prohibits the use of the MIMO scheme when
transmitting a signal to the communication terminal 2.

[0130] As described above, in this modification, in a case where the
reception quality at the communication terminal 2 is good, the use of the
MIMO scheme in transmission to the communication terminal 2 is not
prohibited even when the interference intensity at the communication
terminal 2 is high. This prevents a situation in which the use of the
MIMO scheme when transmitting a signal to the communication terminal 2 is
prohibited although the reception quality at the communication terminal 2
is good. Thus, the base station 1 can use the MIMO scheme more properly,
resulting in a further improvement in communication quality between the
base station 1 and communication terminal 2.

[0131] <Second Modification>

[0132] In a case where a base station 1 uses the MIMO scheme in
transmission, a communication terminal 2 may use the adaptive array
antenna scheme when receiving reference signals for obtaining a channel
matrix from the base station 1. This modification will be described
below.

[0133] FIG. 10 is a flowchart showing an operation of a communication
terminal 2 in a case where a base station 1 uses the MIMO scheme when
transmitting a signal to the communication terminal 2.

[0134] As shown in FIG. 10, in the communication terminal 2 to which a
signal is transmitted from the base station 1 using the MIMO scheme, in
Step s21, a communication unit 25 receives a reference signal from the
base station 1 using the adaptive array antenna scheme. In Step s21,
first, a reception processing unit 241 of the communication unit 25
calculates a plurality of reception weights for controlling the reception
directivity at an array antenna 23, based on the reference signal from
the base station 1. Then, the reception processing unit 241 respectively
sets the calculated plurality of reception weights to the plurality of
reference signals received at a plurality of antennas 22. After that, the
reception processing unit 241 combines the plurality of reference signals
in which the plurality of reception weights have been set, to thereby
generate a combined signal. The combined signal is referred to as an
"array received reference signal". Thus, the communication unit 25
performs null steering and beamforming for the reception directivity at
the plurality of antennas 22, to thereby receive the reference signal
from the base station 1.

[0135] The reception weight may be set to the reference signal used when a
reception weight is obtained, or the reception weight may be set to a
reference signal different from the above-mentioned reference signal.

[0136] In Step s22, next, the reception processing unit 241 estimates a
channel matrix of a desired wave based on the array received reference
signal generated in Step s21, that is, based on the reference signal
received using the adaptive array antenna scheme. The method of
estimating a channel matrix of a desired wave will be described below.

[0137] When y represents the array received reference signal, X represents
a received reference signal vector composed of a plurality of reference
signals respectively received by the plurality of antennas 22, and W
represents a reception weight vector composed of a plurality of reception
weights respectively set in the plurality of reference signals, an array
received reference signal y is expressed by Equation (1) below.

y=W×X (1)

[0138] The received reference signal vector X is expressed by Equation (2)
below.

X=Ht×St+H1×U1+ . . . +HL×UL+N (2)

[0139] St represents a signal vector of a desired wave, and Ui
(1≦i≦L) represents a signal vector of an interference wave.
Ht represents a channel matrix of the desired wave, and Hi represents a
channel matrix of the interference wave. N represents a signal vector of
an internal noise.

[0141] Here, W and N have a weak correlation, and thus, W×N results
in a value approaching zero. W is for directing null for the reception
directivity at the array antenna 23 toward an interference wave, so that
W×Hi×Ui results in a small value. Thus, Equation (3) is
rewritten by Equation (4) below.

y≈W×Ht×St (4)

[0142] The reception processing unit 241 has already known a signal vector
St of a desired wave, namely of an ideal value (original value) of a
reference signal to be transmitted by the base station 1 and can obtain
an array received reference signal y and a reception weight vector W,
whereby the reception processing unit 241 estimates a channel matrix Ht
of a desired wave using those values and Equation (4).

[0143] Next, in Step s23, the reception processing unit 241 performs a
reception process such as a demodulation process on the data signal from
the base station 1 using a channel matrix of a desired wave estimated in
Step s22. As a result, the communication terminal 2 obtains user data and
control data transmitted from the base station 1.

[0144] As described above, in this modification, the communication
terminal 2 performs null steering for the reception directivity at the
plurality of antennas 22 and receives reference signals from the base
station 1, and thus can estimate a channel matrix of a desired wave based
on a reference signal having a low-intensity interference wave included
in the reference signals. A highly accurate channel matrix can be
accordingly obtained. Thus, a reception process is performed on a data
signal from the base station 1 using this channel matrix, so that the
data included in the data signal can be properly obtained. This results
in a further improvement in communication performance between the base
station 1 and communication terminal 2.

[0145] For a combination of the first modification described above and
this modification, the base station 1 desirably judges whether or not the
reception quality at the communication terminal 2 is good based on a
reception error rate for the data obtained by executing Step s23 by the
communication terminal 2 in Step s11 described above.

[0146] <Third Modification>

[0147] In a case where a communication terminal 2 can transmit a signal to
the base station 1 using the MIMO scheme, a base station 1 may determine
whether or not to prohibit the use of the MIMO scheme at the
communication terminal 2 based on the intensity of an interference wave
included in a received signal from the communication terminal 2.

[0148] When the communication terminal 2 transmits a signal using the MIMO
scheme, the base station 1 may avoid the use of the adaptive array
antenna scheme when receiving a signal from the communication terminal 2.

[0149] This modification will be described below. In this modification, a
"communication terminal 2" means a "communication terminal 2 capable of
transmitting a signal using the MIMO scheme" unless otherwise noted. The
MIMO scheme used by the communication terminal 2 is, for example, a
MIMO-SDM.

[0150] FIG. 11 is a diagram showing a configuration of a base station 1
according to this modification. As shown in FIG. 11, in the base station
1 according to this modification, a controller 14 further includes a
reception quality acquisition unit 145.

[0151] The reception quality acquisition unit 145 operates similarly to a
reception quality acquisition unit 242 included in the communication
terminal 2. The reception quality acquisition unit 145 obtains a
reception quality before demodulation and a reception quality after
demodulation for the communication terminal 2 based on a received signal
output from a radio processing unit 11. The reception quality acquisition
unit 145 calculates, for example, a CINR or RSSI as the reception quality
before demodulation. The reception quality acquisition unit 145 also
calculates, for example, an SINR, EVM, or reception error rate as the
reception quality after demodulation. The base station 1 is configured
such that when the reception quality acquisition unit 145 obtains the
reception quality before demodulation and reception quality after
demodulation, a reception processing unit 141 does not receive a received
signal from the communication terminal 2 using the adaptive array antenna
scheme.

[0152] In the base station 1, an interference intensity judgment unit 142
not only judges whether or not the intensity of an interference wave,
which is included in a received signal received from the base station 1
by the communication terminal 2, is high but also judges whether or not
the intensity of the interference wave, which is included in a received
signal received from the communication terminal 2 by the base station 1,
is high. A use prohibition determination unit 143 determines whether or
not to prohibit the use of the MIMO scheme at the base station 1 and also
determines whether or not to prohibit the use of the MIMO scheme in the
communication terminal 2.

[0153] The transmission throughput at the communication terminal 2 is
adjusted by the base station 1. In the base station 1, as in the
adjustment of a transmission throughput of the base station 1 by the
transmission processing unit 140, the reception processing unit 141 first
determines an MCS to be applied to a transmission signal by the
communication terminal 2, based on the reception quality at the base
station 1 for the signal from the communication terminal 2. Then, a
communication unit 15 receives a signal from the communication terminal
2, and then, the reception processing unit 141 judges whether or not the
data included in the received signal has been properly obtained, and then
calculates a reception error rate for the communication terminal 2 based
on the judgment results. In cases where the reception error rate for the
communication terminal 2 is larger than a first threshold or is smaller
than a second threshold (<first threshold), the reception processing
unit 141 adjusts an MCS to be applied to the transmission signal by the
communication terminal 2 as in the case where the transmission processing
unit 140 adjusts an MCS to be applied to a transmission signal that is
transmitted to the communication terminal 2. As a result, in the base
station 1, the reception error rate for received signals from the
communication terminal 2 falls within a predetermined range.

[0154] When an MCS to be applied to a transmission signal by the
communication terminal 2 is determined and changed in the reception
processing unit 141, the communication unit 15 notifies the communication
terminal 2 of this MCS. When being notified the MCS to be used from the
base station 1, the communication terminal 2 applies this MCS to a
transmission signal to the base station 1.

[0155] In a case of starting communication with the communication terminal
2, the base station 1 according to this modification instructs this
communication terminal 2 not to use the MIMO scheme. Then, in a case
where as a result of the reception processing unit 141 adjusting an MCS
to be applied to a transmission signal by the communication terminal 2,
the rank of the MCS reaches the highest rank, a reception error rate for
signals from the communication terminal 2 is smaller than the second
threshold, and the communication terminal 2 is not prohibited from using
the MIMO scheme when transmitting a signal, the communication unit 15
instructs the communication terminal 2 to use the MIMO scheme (MIMO-SDM)
in transmission for further improving the transmission throughput of the
communication terminal 2. In the communication terminal 2, when the use
of the MIMO scheme in transmission is notified from the base station 1, a
communication unit 25 transmits a signal to the base station 1 using the
MIMO scheme.

[0156] In the base station 1, when the reception error rate for signals
from the communication terminal 2 that transmits a signal using the MIMO
scheme becomes larger than the first threshold, the communication unit 15
instructs this communication terminal 2 not to use the MIMO scheme in
transmission. In the communication terminal 2, when it is notified that
the MIMO scheme is not used in transmission from the base station 1, the
communication unit 25 transmits a signal to the base station 1 using the
adaptive array antenna scheme, not using the MIMO scheme. Then, in a case
where the rank of the MCS to be applied to a transmission signal by the
communication terminal 2 is the highest rank, the communication unit 15
instructs this communication terminal 2 to use the MIMO scheme in
transmission when the reception error rate for signals from this
communication terminal 2 becomes smaller than the second threshold.
Hereinafter, the base station 1 and communication terminal 2 operate in a
similar manner.

[0157] As described above, in the base station 1, an MCS to be applied to
a transmission signal by the communication terminal 2 is combined with
whether or not the MIMO scheme (MIMO-SDM) is used when the communication
terminal 2 transmitting a signal, so that the transmission throughput of
the communication terminal 2 is properly adjusted.

[0158] In a case where the use prohibition determination unit 143
prohibits the communication terminal 2 from using the MIMO scheme in
signal transmission, the communication unit 15 does not notify the
communication terminal 2 that the MIMO scheme is used in signal
transmission even if the MCS to be applied to a transmission signal
reaches the highest rank and the reception error rate for signals from
the communication terminal 2 is smaller than the second threshold. In
this case, thus, the transmission scheme is not switched from the
non-MIMO scheme to the MIMO scheme in this communication terminal 2.

[0159] When starting communication with the communication terminal 2, the
base station 1 may first instruct the communication terminal 2 to
transmit a signal using the transmission diversity such as MIMO-STC or
MIMO-SFBC. Then, when the rank of the MCS to be applied to a transmission
signal by the communication terminal 2 reaches the highest rank, the base
station 1 may instruct the communication terminal 2 to use the MIMO-SDM
in signal transmission.

[0160] As in the case where the communication terminal 2 receives a signal
from the base station 1, in the base station 1, when the communication
terminal 2 uses the MIMO scheme in transmission, the reception processing
unit 141 receives a data signal from the communication terminal 2 without
using the adaptive array antenna scheme. When receiving a signal
transmitted from the communication terminal 2 using the MIMO scheme, the
reception processing unit 141 estimates a channel matrix of a
transmission line between the base station 1 and the communication
terminal 2 based on a reference signal from the communication terminal 2.
Then, the reception processing unit 141 performs a reception process on
data signals included in a plurality of received signals output from the
radio processing unit 11 using the estimated channel matrix. Meanwhile,
in a case where the communication terminal 2 does not use the MIMO scheme
in transmission, the reception processing unit 141 receives a data signal
from the communication terminal 2 using the adaptive array antenna
scheme.

[0161] FIG. 12 is a flowchart showing an operation of a wireless
communication system 100 according to this modification, which
corresponds to FIG. 4 described above. As shown in FIG. 12, in Step s31,
in the base station 1, the reception quality acquisition unit 145 obtains
a reception quality before demodulation for a signal from the
communication terminal 2. In Step s32, next, the reception quality
acquisition unit 145 obtains a reception quality after demodulation for
the signal from the communication terminal 2.

[0162] In Step s33, next, the interference intensity judgment unit 142
judges whether or not the intensity of an interference wave included in a
received signal from the communication terminal 2 is high based on the
reception quality before demodulation and reception quality after
demodulation obtained in Steps s31 and s32, as in the case where it is
judged whether or not the interference intensity at the communication
terminal 2 is high (in a similar manner to the method described with
reference to FIG. 5).

[0163] When it is judged in Step s33 that the interference intensity
included in the received signal from the communication terminal 2 is
high, in Step s34, the use prohibition determination unit 143 prohibits
the communication terminal 2 from using the MIMO scheme when transmitting
a signal to the base station 1. Meanwhile, when it is judged in Step s33
that the interference intensity included in the received signal from the
communication terminal 2 is low (is not high), the use prohibition
determination unit 143 does not prohibit the communication terminal 2
from using the MIMO scheme when transmitting a signal to the base station
1.

[0164] As described above, in this modification, if the interference
intensity of the received signal received from the communication terminal
2 by the base station 1 is high, the communication terminal 2 is
prohibited from using the MIMO scheme in signal transmission. This
prevents the base station 1 from performing a reception process on a data
signal from the communication terminal 2 using a less accurate channel
matrix. Therefore, the communication performance between the base station
1 and communication terminal 2 can be improved.

[0165] In the case where the base station 1 adjusts a transmission
throughput at the communication terminal 2 as described above, if the
intensity of an interference wave included in a received signal from the
communication terminal 2 is high, an communication terminal 2 is not
prohibited from using the MIMO scheme in signal transmission. In this
case, in a situation where the interference intensity of the received
signal from the communication terminal 2 is high, an MCS to be applied to
a transmission signal by the communication terminal 2 reaches the highest
rank, and a reception error rate for the received signal from the
communication terminal 2 is smaller than the second threshold, the
transmission scheme of the communication terminal 2 is switched from the
non-MIMO scheme to the MIMO scheme. Upon this, the base station 1
performs a reception process on a data signal from the communication
terminal 2 based on an inaccurate channel matrix. As a result, a
reception error rate for signals from the communication terminal 2
becomes higher in the base station 1.

[0166] When the reception error rate for signals from the communication
terminal 2 becomes higher in the base station 1, the base station 1
switches the transmission scheme of the communication terminal 2 from the
MIMO scheme to the non-MIMO scheme. Upon this, the base station 1
receives a signal from the communication terminal 2 using the adaptive
array antenna scheme. In a case where the base station 1 receives a
signal from the communication terminal 2 using the adaptive array antenna
scheme, the intensity of the interference wave included in the received
signal can be reduced, resulting in a decrease in reception error rate
for signals from the communication terminal 2.

[0167] As a result of a decrease in reception error rate for signals from
the communication terminal 2, the base station 1 again switches the
transmission scheme of the communication terminal 2 from the non-MIMO
scheme to the MIMO scheme. Upon switching of the transmission scheme of
the communication terminal 2 from the non-MIMO scheme to the MIMO scheme,
similarly, a reception error rate for signals from the communication
terminal 2 increases in the base station 1. As a result, the transmission
scheme of the communication terminal 2 is switched from the MIMO scheme
to the non-MIMO scheme. Hereinafter, the base station 1 and communication
terminal 2 repeat similar operations.

[0168] If a communication terminal 2 is not prohibited from using the MIMO
scheme in signal transmission in a case where the intensity of the
interference wave included in a received signal from the communication
terminal 2 is high as described above, in the communication terminal 2,
switching from the non-MIMO scheme to the MIMO scheme and switching from
the MIMO scheme to the non-MIMO scheme frequently occur in an alternate
manner. This destabilizes the operation of the communication terminal 2.
This results in a decrease in communication performance between the base
station 1 and communication terminal 2.

[0169] In this embodiment, in a case where the intensity of an
interference wave included in a received signal from the communication
terminal 2 is high, the base station 1 prohibits the communication
terminal 2 from using the MIMO scheme in signal transmission. Thus, in
the communication terminal 2, switching from the non-MIMO scheme to the
MIMO scheme and switching from the MIMO scheme to the non-MIMO scheme can
be prevented from frequently occurring in an alternate manner. Thus, the
operation of the communication terminal 2 can be prevented from becoming
unstable. This results in a further improvement in communication
performance between the base station 1 and communication terminal 2.

[0170] In this modification, in a case where the communication terminal 2
uses the MIMO scheme in transmission, the base station 1 does not receive
a data signal from the communication terminal 2 using the adaptive array
antenna scheme. This enables the base station 1 to properly receive a
signal transmitted from the communication terminal 2 using the MIMO
scheme. This will be described below.

[0171] When the communication terminal 2 transmits a signal using the MIMO
scheme As in the case where the base station 1 transmits a signal using
the MIMO scheme, a plurality of antennas 22 are configured not to
transmit a plurality of different reference signals using the same radio
resources (same frequency band and same transmission time slot) such that
the base station 1 can properly estimate a channel matrix using the
reference signals. Meanwhile, as to the data signal to be transmitted
from the communication terminal 2 using the MIMO scheme, the plurality of
antennas 22 transmit a plurality of different data signals using the same
radio resources.

[0172] Here, in a case where the base station 1 properly receives, using
the adaptive array antenna scheme, a data signal transmitted by the
communication terminal 2 using the MIMO scheme, for each of a plurality
of antennas 22 of the base station 1, the base station 1 needs to direct
a beam toward a data signal transmitted from one antenna 12 and direct
null toward a data signal transmitted from the other antenna 22.

[0173] However, in the communication terminal 2, the mode of transmitting
a reference signal and the mode of transmitting a data signal differ from
each other as described above when the MIMO scheme is used. For this
reason, even if the base station 1 performs null steering for the
reception directivity at the plurality of antennas 12 based on the
reference signal transmitted from the communication terminal 2, for each
of a plurality of antennas 22 included in the communication terminal 2,
the base station 1 cannot direct a beam toward a data signal transmitted
from one antenna 22 and direct null toward a data signal transmitted from
the other antenna 22. Thus, the base station 1 cannot properly receive,
using the adaptive array antenna scheme, a data signal transmitted from
the communication terminal 2 using the MIMO scheme.

[0174] In this modification, therefore, the base station 1 is configured
not to receive a data signal from a communication terminal 2 using the
adaptive array antenna scheme in a case where the communication terminal
2 uses the MIMO scheme in transmission to the base station 1. This
enables the base station 1 to properly receive a signal transmitted from
the communication terminal 2 using the MIMO scheme. This results in a
further improvement in communication performance between the base station
1 and communication terminal 2.

[0175] As in the first modification, also in this modification, the base
station 1 may prohibit a communication terminal 2 from using the MIMO
scheme in transmission in a case where the interference intensity of the
received signal from the communication terminal 2 is high and also in a
case where the reception quality for a signal from the communication
terminal 2 is not good (is poor). Then, even in a case where the
interference intensity of the received signal from the communication
terminal 2 is high, the base station 1 does not need to prohibit the
communication terminal 2 from using the MIMO scheme in transmission when
the reception quality for a signal from the communication terminal 2 is
good.

[0176] As described above, even in a case where the interference intensity
of the received signal from a communication terminal 2 is high, the
communication terminal 2 is not prohibited from using the MIMO scheme in
transmission when the reception quality for a signal from the
communication terminal 2 is good. This prevents a situation in which a
communication terminal 2 is prevented from using the MIMO scheme in
signal transmission even though the reception quality for a signal from
the communication terminal 2 is good. Thus, the communication terminal 2
can more properly use the MIMO scheme, resulting in a further improvement
in communication quality between the base station 1 and communication
terminal 2.

[0177] As in the second modification, also in this modification, in a case
where the communication terminal 2 uses the MIMO scheme in transmission,
the base station 1 may use the adaptive array antenna scheme when
receiving a reference signal for obtaining a channel matrix from the
communication terminal 2. In this case, first, the reception processing
unit 141 of the base station 1 calculates a plurality of reception
weights for controlling the reception directivity at the array antenna 13
based on a reference signal from the communication terminal 2. Then, the
reception processing unit 141 respectively sets the calculated plurality
of reception weights to the plurality of reference signals received by a
plurality of antennas 12. After that, the reception processing unit 141
combines the plurality of reference signals in which the plurality of
reception weights have been set, to thereby generate an array received
reference signal. As a result, the communication unit 15 performs null
steering and beamforming for the reception directivity at the plurality
of antennas 12, to thereby receive a reference signal from the
communication terminal 2.

[0178] After that, the reception processing unit 141 estimates a channel
matrix of a desired wave in a transmission line between the base station
1 and communication terminal 2 based on the generated array received
reference signal, that is, based on the reference signal received using
the adaptive array antenna scheme. Equation (4) above can be used in
estimating a channel matrix of a desired wave.

[0179] After estimating a channel matrix of a desired wave, the reception
processing unit 141 performs a reception process such as a demodulation
process on a data signal from the communication terminal 2 using the
channel matrix. This enables the base station 1 to obtain user data and
control data transmitted from the communication terminal 2.

[0180] As described above, the base station 1 performs null steering for
the reception directivity at a plurality of antennas 12 and receives
reference signals from the communication terminal 2, to thereby estimate
a channel matrix of a desired wave based on a reference signal having the
interference wave whose intensity is high included in the received
signals. Thus, a highly accurate channel matrix can be obtained, and a
reception process can be performed on data signals from the communication
terminal 2 using this channel matrix. As a result, the data included in
the data signals from the communication terminal 2 can be properly
obtained.

[0181] While the invention has been described in detail, the foregoing
description is in all aspects illustrative and not restrictive. It will
be appreciated that numerous modifications unillustrated herein can be
made without departing from the scope of the present invention.

REFERENCE SIGNS LIST

[0182] 1 base station

[0183] 2 communication terminal

[0184] 12, 22
antenna

[0185] 15, 25 communication unit

[0186] 100 wireless
communication system

[0187] 142 interference intensity judgment unit

[0188] 143 use prohibition determination unit

[0189] 144 reception
quality judgment unit

[0190] 145, 242 reception quality acquisition unit

Patent applications by Takeo Miyata, Yokohama-Shi JP

Patent applications by Kyocera Corporation

Patent applications in class Plural channels for transmission of a single pulse train

Patent applications in all subclasses Plural channels for transmission of a single pulse train